1. Field of the Invention
The invention relates to a method of producing a sulfide solid electrolyte material that generates an extremely small amount of hydrogen sulfide.
2. Description of the Related Art
The rapid diffusion of information-related devices and communication devices such as personal computers, video cameras, cell phones and the like in recent years has been accompanied by efforts directed at developing batteries to be used as power sources in the foregoing devices. Meanwhile, high-output and high-capacity batteries for electric automobiles and hybrid automobiles are the subject of ongoing research in the automotive industry. At present, lithium batteries are predominant among various types of battery, thanks to their high energy density.
Current commercially available lithium batteries use electrolyte solutions that contain combustible organic solvents. As a result, safety devices that suppress rises in temperature during short-circuits must be fitted to the battery, and also improvements in production and materials are called for in order to prevent short-circuits. By contrast, lithium batteries that are all-solid-state batteries, by changing the electrolyte solution into a solid electrolyte layer, employ no combustible organic solvent inside the battery, and arguably make for simpler safety devices, while being superior in terms of production costs and productivity. Sulfide solid electrolyte materials are conventional solid electrolyte materials that are employed in such solid electrolyte layers.
Sulfide solid electrolyte materials are useful for achieving higher battery outputs thanks to their high lithium (Li) ion conductivity, and research is being conducted on related technologies. For instance, Nobuya Machida et al., “Mechano-chemical Synthesis of Lithium Ion Conducting Materials in the System Li2O—Li2S—P2S5”, J. Jpn. Soc. Powder Metallurgy Vol. 51, No. 2, 91-97 disclose a glassy Li ion conductive material wherein part of Li2S in 75Li2S-25P2S5 is replaced by Li2O. Also, R. Prasada Rao et al., “Oxysulfide glasses xLi2O-(1-x)(0.6Li2S-0.4P2S5)”, Journal of Power Sources 159 (2006) 258-262, discloses a glassy Li ion conductive body represented by 40Li2O-36Li2S-24P2S5 (in this composition, the material satisfies x=40 in xLi2O-(1-x)(60Li2S-40P2S5)) and that is produced by mechanical milling. In both technologies, a Li ion conductive body is produced in one single vitrification process (mechanical milling process).
Sulfide solid electrolyte materials are advantageous in terms of high Li ion conductivity, but, on the other hand, are problematic in that they generate hydrogen sulfide when coming into contact with water (such as moisture, likewise hereafter). Against this background, the inventors have found that the amount of generated hydrogen sulfide can be reduced by adjusting the composition of a sulfide solid electrolyte material to an ortho composition. The term ortho composition denotes ordinarily the most highly hydrated oxoacid from among the oxoacids that are obtained through hydration of one same oxide. In the sulfide solid electrolyte material having Li2S, however, the ortho composition denotes a composition having a crystal composition with the greatest amount of Li2S added among the sulfides. In a Li2S—P2S5 material, for instance, Li3PS4 corresponds to the ortho composition, such that a sulfide solid electrolyte material of ortho composition is obtained when mixing starting materials in a proportion Li2S:P2S5=75:25, on a molar basis. Although a sulfide solid electrolyte material having an ortho composition has a lower amount of generated hydrogen sulfide than a sulfide solid electrolyte material other than of ortho composition, the sulfide solid electrolyte material releases nonetheless small amounts of hydrogen sulfide, and drops in Li ion conductivity have been observed. Therefore, hydrogen sulfide generation must be reduced further still in order to increase the stability of the sulfide solid electrolyte material.